1. Field of the Invention
The present invention relates to: an A/D conversion apparatus for converting an analog signal to a digital signal; a solid-state image capturing apparatus, in which analog pixel signals (image capturing signals) from a plurality of semiconductor elements are A/D converted in a column parallel by the A/D conversion apparatus and are subsequently processed with various image processing to obtain color image signals, the semiconductor elements being for performing a photoelectric conversion on and capturing an image of incident light from a subject; and an electronic information device, such as a digital camera (e.g., a digital video camera or a digital still camera), an image input camera (e.g., a monitoring camera), a scanner, a facsimile machine, a television telephone device and a camera-equipped cell phone device, including the solid-state image capturing apparatus as an image input device used in an image capturing section thereof.
2. Description of the Related Art
There is a conventional technique described in Reference 1, for example, as an A/D conversion method associated with a conventional solid-state image capturing apparatus. According to Reference 1, in a solid-state image capturing apparatus with a conventional built-in A/D conversion apparatus, it is possible to reduce noise and power consumption due to the driving of digital systems.
FIG. 18 is a block diagram illustrating an exemplary configuration of essential parts of a conventional solid-state image capturing apparatus disclosed in Reference 1.
In FIG. 18, a conventional solid-state image capturing apparatus 100 is provided with a plurality of pixels 101 functioning as a plurality of sensing elements for performing a photoelectric conversion on and capturing an image of incident light from a subject. These pixels 101 are arranged in rows and columns (in a matrix) to constitute a pixel section (pixel area) 102. The pixels 101 as sensing elements are constituted of, for example, a CCD sensor, a CMOS sensor or a near infrared ray sensor, constituted of photodiodes (PD), or a sensor that converts infrared rays into heat and further into electric signals. It goes without saying that the present invention is not limited to the examples of these sensors, but the examples may further include a sensing element, such as a pressure sensor and the like.
In FIG. 18, one gray code counter 103 is provided for the solid-state image capturing apparatus 100. The output of the gray code counter 103 is connected via a common signal line 104 to an A/D converter 105, which is constituted of a comparator 106 and a digital memory 107. One A/D converter 105 is provided for each column. Herein, three columns are illustrated in the example; however, the number of columns is a design choice and the present invention is not limited to this example. In addition, there may be a case where one A/D converter is provided for each column, one A/D converter is provided for two or more columns, or two or more A/D converters are provided for each column.
A digital/analog converter (DAC) 108 is connected to the gray code counter 103 via a gray code/binary converter 109, and they are operated in synchronism. At an A/D conversion, a triangle wave is output from the DAC 108 as a reference signal. In the comparator 106, data 110 from the pixels 101 is compared with the triangle wave. The digital memory 107 is selectively connected to a horizontal digital output line 111, and the horizontal digital output line 111 is connected to an output buffer 113 via a gray code/binary code converter 112. The above described members are formed on a semiconductor chip, and are output outside a sensor chip via the output buffer 113.
Each digital memory 107 is connected to the horizontal digital output line 111 via switches (not shown). By switching on each of the switches at a time, the output is made selectively from the digital memory 107 to the horizontal digital output line 111. A method for successively switching on switches includes a method for decoding an address using a switch pulse, a method for successively switching on switches for each column using a digital shift register, and the like. As described above, the selective output is successively performed from each digital memory 107 to the horizontal digital output line 111.
With the configuration described above, the plurality of pixels 101 are, first selectively scanned one row at a time, and each of output pixel signals is input into one of input terminals of the comparator 106.
Thereafter, the triangle signal as a reference signal which is synchronized from the DAC 108 to the gray code counter 103 is input into the other input terminal of the comparator 106. At the same time, a value of the gray code counter 103 is distributed to all the digital memories 107. The comparator 106 reverses an output signal at a time corresponding to the magnitude (output value) of an output pixel signal from the pixels 101. Using a reversed signal as a trigger, the digital memory 107 retains a count value in accordance with the reversed time instances, from the gray code counter 103 to perform the A/D conversion.
This A/D conversion result is sent to the gray code/binary code converter 112 via the horizontal digital output line 111. The gray code/binary code converter 112 converts the result into a binary code that can be handled by other digital devices, such as a CPU. The converted binary code is output from the output buffer 113 to the outside as data for each pixel 101.
The gray code counter 103 starts counting at a time when a triangle voltage of the triangle signal is started to be applied to the comparator 106, which is at a preceding stage of the digital memory 107. The output of the comparator 106 is reversed as a result of comparing the triangle voltage and the output value of the output pixel signal of the pixels 101, and a count value corresponding to the output, of the gray code counter 103 is output and retained by the digital memory 107. It is particularly important for an image sensor to include the DAC 108 built therein, and low noise characteristics are particularly important for such an image sensor because the image sensor deals with minute analog voltage. It is required to perform A/D conversion on analog pixel signals (image capturing signals) from the pixels 101 with low noise.
In the future, further noise reduction will be required in order to decrease the period of the triangle signal and decrease the period of the A/D conversion. Accordingly, it will be required to build the DAC 108 in the sensor and connect the output terminal of the DAC 108 to the A/D converter 105 in a shortest possible distance.
By building inside the gray code/binary code converter 112 for converting a gray code into a binary code and the DAC 108, the image sensor is capable of, while obtaining advancements of the gray codes, such as effects of low power consumption, low noise and the like, suppressing, to the minimum, disturbance between the output of the DAC 108 and the input section of the comparator 106 to perform accurate A/D conversion.    Reference 1: Japanese Laid-Open Publication No. 2005-347931